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Abstract:

A method for decontaminating at least one object contained in a chamber,
the method including a succession of alternated steps of lowering and
increasing the pressure in the chamber.

Claims:

1. A method for decontaminating at least one object contained in a
chamber, the method comprising a succession of alternated steps of
lowering and increasing the pressure in the chamber, wherein, in pressure
increasing steps, a gas previously heated to a temperature greater than
the ambient temperature is injected into the chamber.

2. The method of claim 1, wherein said at least one object is a
semiconductor wafer.

4. The method of claim 1, wherein the pressure lowering and increasing
steps are repeated from 3 to 15 times each.

5. The method of claim 1, wherein in pressure lowering steps, the
pressure in the chamber is lowered down to a low value smaller than
10.sup.-3 mPa.

6. The method of claim 1, wherein in pressure increasing steps, the
pressure in the chamber is increased up to a high value ranging from 30
to 100 percent of the atmospheric pressure.

7. The method of claim 1, wherein in pressure increasing steps, nitrogen
is injected into the chamber.

8. The method of claim 1, wherein each cycle comprising a pressure
lowering step and a pressure increasing step consecutive to the lowering
step, has a duration ranging from 3 to 10 minutes.

9. The method of claim 1, wherein at an end of each pressure lowering
step, the pressure in the chamber is maintained at a low value for a time
interval shorter than 2 minutes.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of French patent
application number 10/52977, filed on Apr. 20, 2010, entitled "METHOD FOR
DECONTAMINATING SEMICONDUCTOR WAFERS," which is hereby incorporated by
reference to the maximum extent allowable by law.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for decontaminating
semiconductor wafers. It more specifically aims at the decontamination of
wafers likely to have adsorbed corrosive gases in steps of forming of
conductive copper or aluminum interconnection tracks and vias.

[0004] 2. Discussion of the Related Art

[0005] Conventionally, integrated circuit manufacturing methods comprise
steps of conductive copper or aluminum interconnection track and via
forming, at the surface of semiconductor wafers, for example, silicon
wafers. The forming of such tracks and vias especially comprises
successive steps of deposition and etching of metal layers and of
insulating layers. In the etch steps, especially the plasma etch steps,
various contaminating elements may be produced and adsorbed, for example,
in the insulating layers of the interconnection stack. The presence of
such contaminating elements in the wafers may result, later on, in a
deterioration of the integrated circuits.

[0006] A step of wafer decontamination after the forming of the conductive
tracks and vias is currently provided.

[0007] A decontamination method comprising placing the wafers in vacuum
for a relatively long time to extract the contaminating elements adsorbed
during etch operations has been provided. To achieve this, at the end of
the manufacturing, the wafers are placed in transfer and processing
containers, each container containing a large number of wafers. Such
containers are generally designated as "pods" or FOUP ("Front Opening
Unified Pod") in the art. One or several pods are placed in a
decontamination chamber. The decontamination chamber is then set to a
pressure much lower than the atmospheric pressure, for example, a
pressure lower than 10-3 mPa. The chamber will be said to be
vacuumized.

[0008] A disadvantage of such a method is that, to obtain a satisfactory
result, the pods should stay in the decontamination chamber for a long
time, for example, on the order of 60 min.

[0009] It would be desirable to have faster wafer decontaminating means.

[0010] To accelerate the contaminating element elimination process, it has
been suggested to heat the wafers during the decontamination. Indeed, the
diffusion speed of contaminating elements increases along with
temperature. However, in practice, it is very difficult to heat the
wafers satisfactorily. Indeed, due to the very low pressure in the
decontamination chamber, convection heating is impossible. Further, the
arrangement of the wafers, which are stacked in pods, forbids heating by
infrared radiation. Similarly, conduction heating is not very efficient
since only a small portion of the wafer surface is in direct contact with
the pods.

[0011] A decontamination method comprising placing the wafers in storage
cabinets under a low-pressure flow of nitrogen or another inert gas has
also been provided. The storage under a nitrogen flow especially enables
avoiding any corrosion due to the contaminating elements. However, the
decontamination time is then very long. Further, such a method induces
unwanted nitrogen consumption.

SUMMARY OF THE INVENTION

[0012] Thus, an object of an embodiment is to provide a method for
decontaminating semiconductor wafers at least partly overcoming some of
the disadvantages of prior art solutions.

[0013] Another object of an embodiment is to provide such a method
enabling a faster wafer decontamination than existing solutions.

[0014] Another object of an embodiment is to provide such a method which
is easy to implement, and especially easy to implement by using existing
decontamination equipment.

[0015] Thus, an embodiment provides a method for decontaminating at least
one object contained in a chamber, this method comprising a succession of
alternated steps of lowering and increasing the pressure in the chamber.

[0016] According to an embodiment, said at least one object is a
semiconductor wafer.

[0017] According to an embodiment, in pressure increase steps, a gas
previously heated to a temperature greater than the ambient temperature
is injected into the chamber.

[0018] According to an embodiment, this temperature ranges between 40 and
90° C.

[0019] According to an embodiment, the pressure lowering and increase
steps are repeated from 3 to 15 times each.

[0020] According to an embodiment, in pressure lowering steps, the
pressure in the chamber is lowered down to a low value smaller than
10-3 mPa.

[0021] According to an embodiment, in pressure increase steps, the
pressure in the chamber is increased up to a high value ranging from 30
to 100 percent of the atmospheric pressure.

[0022] According to an embodiment, in pressure increase steps, nitrogen is
injected into the chamber.

[0023] According to an embodiment of the present invention, each cycle
comprising a pressure lowering step and a pressure increase step,
consecutive to the lowering step, has a duration ranging from 3 to 10
minutes.

[0024] According to an embodiment, at the end of each pressure lowering
step, the pressure in the chamber is maintained at a low value for a time
interval shorter than 2 minutes.

[0025] The foregoing objects, features, and advantages will be discussed
in detail in the following non-limiting description of specific
embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a cross-section view very schematically showing an
example of a semiconductor wafer decontamination chamber;

[0027] FIG. 2 is a diagram schematically showing steps of an embodiment of
a semiconductor wafer decontamination method; and

[0028]FIG. 3 is a diagram schematically showing an alternative embodiment
of the method of FIG. 2.

DETAILED DESCRIPTION

[0029] For clarity, the same elements have been designated with the same
reference numerals in the different drawings and, further, FIG. 1 is not
drawn to scale.

[0030] FIG. 1 is a cross-section view very schematically showing an
example of a semiconductor wafer decontamination chamber 1. This
equipment is conventionally used to implement the above-mentioned
decontamination method, where the wafers are placed in vacuum for a
relatively long time. Chamber 1 is a tight enclosure into which emerge a
gas mixture intake nozzle 3 and injection nozzle 5. Nozzle 3 is, for
example, connected to a vacuum pump (not shown). Nozzle 5 enables
injecting, into the chamber a gas, for example, air, to restore a
pressure close to the atmospheric pressure at the end of the
decontamination process. Nozzles 3 and 5 are provided with tight closing
valves (not shown). In this example, chamber 1 contains a pod 7 in which
semiconductor wafers 9 are arranged. In pod 7, a support 11 enables to
maintain wafers 9 parallel to one another and facing each other two by
two. Thus, the wafers, for example by the number of 25, are stacked, with
a free space separating the wafers from one another. Pod 7 comprises
openings enabling the pressure within pod 7 to balance with the pressure
in chamber 1.

[0031] The present inventors have observed that an alternation of steps of
lowering and increase of the pressure in the decontamination chamber
results in a faster elimination of the contaminating elements than a
maintaining of the wafers at constant pressure, even very low. This is
especially due to the fact that pressure variations in the
decontamination chamber cause an increase in the contaminant
concentration gradient, thus promoting the diffusion contaminating
elements.

[0032] FIG. 2 is a diagram schematically showing steps of an example of a
method for decontaminating semiconductor wafers. As described hereabove,
the wafers are arranged in pods, and one or several pods are placed in a
decontamination chamber of the type described in relation with FIG. 1.
Initially, the pressure in the decontamination chamber is approximately
equal to the atmospheric pressure.

[0033] In a step 21, pressure P in the decontamination chamber is taken
down to a low value P0, for example, lower than 10-3 mPa. The
pressure in the decontamination chamber may be maintained at low value P0
for some time, for example, from 0 seconds to 2 minutes.

[0034] In a step 23 following step 21, pressure P in the decontamination
chamber is taken up to a high value P1 greater than P0. As an example,
high value P1 may range between 30 and 100% of the atmospheric pressure.
The restoring of pressure P to a value greater than P0 may be obtained by
injecting a gas mixture, for example, air, nitrogen, or another inert gas
or gas mixture (argon, helium, etc.), via nozzle 5.

[0035] When high value P1 has been reached, the pressure in the
decontamination chamber is lowered back to P0 (step 21). Steps 21 and 23
are alternately repeated N times, N being an integer, for example ranging
between 3 and 15. At the end of the process, in a step 25, pressure P in
the decontamination chamber is taken back to the atmospheric pressure
Patm.

[0036] Although they comprise pressure-balancing ports, pods 7 (FIG. 1)
are provided to maintain the wafers in a relatively confined atmosphere.
Indeed, such pods are especially used, in the transfer of the wafers from
one piece of equipment to another, to protect the wafers against possible
contaminations by outer particles (dust, etc.). The pressure variations
in the decontamination chamber should thus be progressive and
sufficiently slow to avoid that the pods explode or implode. As an
example, each cycle of lowering/restoring of the pressure in the chamber
may last from 3 to 10 minutes, the number of cycles being selected
according to the cycle duration so that the total decontamination time is
much shorter than one hour. To be able to more rapidly lower/restore the
pressure, it may be provided to use pods having wide openings, or to
maintain the pods open. In this case, it will be ascertained that
parasitic particles do not risk contaminating the wafers.

[0037] An advantage of the provided method is that it enables
decontaminating the wafers faster than when they are maintained in vacuum
at constant pressure. Another advantage of this method is that it can
easily be implemented by using a conventional vacuum decontamination
chamber, of the type described in relation with FIG. 1.

[0038] The present inventors have observed that the method described in
relation with FIG. 2 results in a decrease on the order of 40% of the
decontamination time with respect to the conventional solution where the
wafers are maintained in vacuum, at constant pressure and temperature.

[0039] As an example, number N of pressure lowering/restoring cycles may
be set to 5, low pressure P0 may be equal to 5*10-4 mPa, high
pressure P1 may be equal to the atmospheric pressure, and the duration of
each cycle may be equal to 7 min, including maintaining of the chamber at
low pressure P0 for 1 min. With such parameters, resulting in a total
decontamination time of 35 min, the present inventors have obtained a
decontamination level equivalent to that obtained by maintaining the
wafers in vacuum for 60 min.

[0040]FIG. 3 is a diagram schematically showing an alternative embodiment
of the decontamination method described in relation with FIG. 2. As in
the method of FIG. 2, initially, the pressure in the decontamination
chamber is approximately equal to the atmospheric pressure. Further,
temperature T in the decontamination chamber is approximately equal to
the ambient temperature (temperature outside of the decontamination
chamber), that is, for example, ranging between 15 and 30° C.

[0041] In a step 31, corresponding to step 21 of FIG. 2, pressure P in the
decontamination chamber is taken down to a low value P0.

[0042] In a step 33, following step 31, corresponding to step 23 of FIG.
2, pressure P in the decontamination chamber is taken back to a high
value P1 greater than P0. In this embodiment, the gas, for example air or
nitrogen, introduced into the chamber to increase pressure P, has been
previously heated up to a temperature T1 greater than the ambient
temperature. As an example, temperature T1 ranges between 40 and
90° C. It should be noted that temperature T1 may take any other
adapted value. This value will be preferably selected to be relatively
high, but of course sufficiently low to avoid damaging the elements which
are desired to be decontaminated.

[0043] As in the method of FIG. 2, steps 31 and 33 are alternately
repeated N times. At the end of the process, in a step 35, pressure P in
the decontamination chamber is taken back to atmospheric pressure Patm.

[0044] An advantage of this embodiment is that it enables heating the
semiconductor wafers by convection, by introducing a hot gas into the
chamber on each occurrence of pressure restoring step 33. This enables
accelerating the diffusion of the contaminating gases. Such a heating of
the wafers is, as discussed previously, impossible to obtain with the
conventional method where the wafers are maintained in vacuum for a long
time.

[0045] Specific embodiments of the present invention have been described.
Various alterations and modifications will occur to those skilled in the
art.

[0046] In particular, a method for decontaminating semiconductor wafers
having adsorbed contaminating elements after chemical etch operations has
been described herein. The present invention is not limited to this
specific case. It will be within the abilities of those skilled in the
art to implement the provided method to decontaminate any device (wafer,
container, wafer transport box, photolithography mask, or other) that may
have adsorbed contaminating elements, whatever the contamination source.

[0047] Further, the provided method comprises an alternation of steps of
pressure decrease in the decontamination chamber down to a low pressure
P0, and of pressure increase in the decontamination chamber up to a high
pressure P1 greater than P0. The values mentioned hereabove for low and
high pressures P0 and P1 have been given as an example only. The present
invention is not limited to these specific cases. It should be noted
that, should the equipment allow it, low pressure P0 may be lower than
10-4 mPa and high pressure P1 may be greater than the atmospheric
pressure. It may further be chosen to modify low and high values P0 and
P1 of the pressure in the chamber each time the cycle is repeated.

[0048] Similarly, the above-mentioned numerical values for temperature T1
to which the decontamination chamber is heated, for number N of cycles,
for the cycle duration, and for the time for which the chamber is
maintained at low pressure P0, have been given as an example only.

[0049] Such alterations, modifications, and improvements are intended to
be part of this disclosure, and are intended to be within the spirit and
the scope of the present invention. Accordingly, the foregoing
description is by way of example only and is not intended to be limiting.
The present invention is limited only as defined in the following claims
and the equivalents thereto.